The present invention relates to an engine with a supercharger, and in particular relates to an engine equipped with a turbo charger which performs a dynamic-pressure supercharging without using a manifold.
In the engine equipped with the turbo charger, the dynamic-pressure supercharging to enhance a drive force of a turbine wheel through a direct injection of the exhaust gas from each cylinder to the turbine wheel has recently become more popular than the static-pressure supercharging in which the exhaust gas from the plural cylinders is collected by an exhaust manifold and then injected toward the turbine wheel.
Japanese Patent Laid-Open Publication No. 2008-31942 from the present applicant, for example, proposed the engine with a supercharger, in which exhaust passages of the engine which connect to the exhaust ports of the cylinders are divided into two groups, each group being provided for the cylinders, ignition timings of which are not adjacent to each other, and one of the groups provided for two cylinders which are positioned closely to each other having a relatively small passage volume is connected to a relatively outside position of the turbine wheel, while the other of the groups provided for two cylinders which are positioned far away from each other having a relatively large passage volume is connected to a relatively inside position of the turbine wheel. Thereby, the exhaust gas having a greater flowing speed is made hit against an outside portion of the turbine wheel, so that the high supercharging can be provided effectively.
According to the engine of the above-described publication, however, since the exhaust gas from all of the cylinders is collected for the single turbo charger, there is a problem in that in case the mixture gas becomes lean, the temperature of the exhaust gas decreases, so that the rotational speed of the turbine wheel becomes so small especially in a low engine-speed area that a properly great supercharging pressure may not be obtained.